Increasing Output for Large Stainless Steel Sheet Surface Polishing

Q: We are having a tough time finding a successful approach to increasing output for large stainless steel sheet surface polishing. What advice do you have to help us achieve that?

A: The best answer would be to give you a real-life example. Striving to increase output, a large stainless steel sheet manufacturer had a strategy in place to increase the table speed of their three-headed (top heads), wide-belt sanding units. However, they had very little success due to surface finish and mechanical issues, so they sought a better solution.

Upon the first review of their overall process, it was observed that due to the initial setup of the equipment, which they had done according to the machine builder’s instructions, they were processing 25-32 “break-in” sheets each day. This process was implemented to allow them to get an entire day of running with the same three wide belts. The goal was to reduce the abrasive belt cost by using overly aggressive grits in the initial setup, allowing them to “break-in” and then run an acceptable roughness average (Ra) after that break-in period.

Valued at $500 per sheet, this would be a potential loss of sales of up to $16,000 per day. Perhaps, at least equally important, would be the loss of 32 sheets daily or 160 sheets weekly in overall output. With their current output, this amounted to nearly a day of production lost each week. Due to today’s exceedingly tight production schedules and competitive environment, there are very few organizations that can afford to maintain this loss, either financially or by delayed customer orders.

To provide the solution they needed — an increase in output on large stainless steel sheets — a different approach (from the one recommended by other abrasive suppliers and the machine builder) was taken. Based on the Ra range needed, no higher than 25 Ra and no lower than 10 Ra, a process was created to achieve this range much quicker and with a reduction in “break-in” sheets.

A 100-grit, X-weight silicon carbide cloth belt was placed on the first head of the sanding units to remove all surface defects (nicks, cut, dings and more), but without leaving a scratch pattern that is too deep or too wide, which would need to be reduced later.

After the initial rough-cut process, heads two and three used paper or cloth X-weight belts with 120 grit silicon carbide grain. Either backing works, but the cloth appeared to provide a slightly longer life and sustained a higher head pressure and amperage. The use of these grits, which were a couple steps finer than what was previously used, allowed the manufacturer to go from the 32 break-in sheets to 12 setup sheets in one run and then 10 in the next.

This translated to a reduction in waste of at least 20 setup sheets per day or an output of $10,000 in additional product. For the week, it added up to a $50,000 value improvement. This, on top of the elimination of massive material handling efforts that the operators were performing, resulted in substantial productivity improvements. In the original process, the sheets also needed to be moved from the exit end of the machine back to the start end. This produced an increased opportunity to run beyond what was considered a standard daily production of 200 sheets.

Further examining the new process, a less than optimal scenario was reviewed. Production numbers were based on running only three-quarters of a shift each day. For the week, there were seven belt changes versus the best scenario of having only five changeovers. Using cost numbers of $200 per belt for head No. 1 (cloth belt) and $135 each for belts on heads two and three, the changeover cost was $470 per set. Operating a three-quarters shift, the weekly cost was $3,290, compared to the cost of $2,350 if the set of belts last a full shift. This was an increase in abrasive cost of $940 per week, along with the additional cost of 20 break-in sheets, which was $10,000 for the week.

Even this less optimal scenario example realizes a weekly value improvement of $40,000 in stainless steel sheet production with a minimal increase in abrasive costs of $940. This was a net cost value improvement of $39,060 for a normal full week of production on the finished stainless steel sheets. Also, this was possible with no compromise in the overall sheet finish quality and with an acceptable finish achieved much quicker using the finer silicon carbide grit setup versus using the coarse belt.

The use of the coarser grits showed very little overall value in finish and was certainly not efficient in terms of time and output. Clearly, by correcting the grit the manufacturer eliminated wasted time through setup sheet reduction and also improved utilization. Its large, custom-made finishing machine is now running saleable sheets versus rerun sheets. This is a multitiered improvement.

Mike Shappell is a senior application engineer at Norton Saint-Gobain. Visit nortonabrasives.com.